Digesting Food is Like Playing with Lego Blocks

Your stomach and small intestine, working in tandem, constitute the ultimate recycling center. All manner of things come sliding down that long chute from the mouth. When this material lands in the stomach, then the countdown begins. The industrious crew of enzymes is given a limited amount of time to disassemble all that junk into its component parts. Whatever material cannot be taken apart within a couple of hours will get flushed down to the large intestine for disposal. But anything that is completely disassembled will have access to a secret exit — and will disappear through the walls of the small intestine into the bloodstream.

The strange little secret is that your body insists on tearing all of those food molecules apart before putting them back together. It’s as if your body purchased a nice 3-bedroom house, and then tore it apart into a pile of bricks and lumber — just to use the materials to build a new 3-bedroom house, in a different layout and style. However, your body does not insist on making its own components from scratch — the equivalent of the bricks and lumber. In fact, it doesn’t know how to make most of the basic components. Therefore it deconstructs the food into the basic building blocks, and then it builds new things from those pieces.

The three main types of nutritious materials in your food are carbohydrates, fats, and proteins. All of these consist of relatively complex molecules. But each of these molecules was originally constructed by a plant or animal by stringing together a series of simpler molecules, like creating a necklace from a set of beads, or a toy castle from Lego blocks. When you swallow such a molecule, your body insists on taking it all apart, back to the original Lego blocks, before it is willing to accept any of it. Individual, disassembled blocks are given permission to leave the small intestine by way of the secret exit (disappearing through the wall), but clumps of Legos that are still stuck together will get flushed away when the timer runs out.

So let’s look at that those three main types of materials — carbohydrates, fats, and proteins — to see what the individual Lego blocks look like.

Carbohydrates

Carbohydrates are sugars and starches. The Lego blocks for carbohydrates are little ring-shaped molecules called simple sugars or monosaccharides. Some have names that you have probably heard of, such as glucose and fructose. Another simple sugar is called galactose. Other sugars and starches are made by stringing together a chain of these simple sugars.

Each molecule of a simple sugar consists of six atoms of carbon, six atoms of oxygen, and around 12 atoms of hydrogen. The “backbone” of the molecule is the sequence of six connected carbon atoms, which typically loops around to form a ring. Because all simple sugars have this same general structure, they are all quite similar to one another.

Sucrose, which is the ordinary table sugar that we get from sugar cane and sugar beets, is a larger molecule consisting of two simple sugars — two saccharide rings — that are linked together. A sugar molecule that includes two saccharide rings is called a disaccharide. In the case of sucrose, one of the rings is glucose and the other ring is fructose. In the case of lactose, a two-ring sugar found in milk, one ring is glucose and the other is galactose.

Longer combinations are also possible, consisting of dozens, hundreds, or even thousands of saccharide rings. These long saccharide chains don’t taste sweet, so we call them starches instead of sugars. These complex carbohydrates, also called polysaccharides, are especially plentiful in our starchy foods, such as bread, potatoes, and rice.

When we eat sugars and starches, our bodies insist on breaking them down into the basic Lego blocks — the simple one-ring sugars, such as glucose, fructose, and galactose. These are the only carbohydrates that are allowed to use the secret exit from the small intestine. After exiting, the sugar molecules enter the blood system, where they can be transported all around the body. These molecules serve as the principal fuel supply for our muscles and for all of the other cells in our bodies. However, the body needs to maintain a careful balance regarding the amount of sugar floating around in the blood — not too little and not too much. A limited amount can be stored as glycogen in the liver and muscles. (Glycogen is a kind of starch consisting of a lot of glucose molecules stuck together in a highly branched structure.) But beyond that, any excess sugar is converted to fat and stored away in fat cells for future use.

Even though sugars and starches both become simple sugars before entering the bloodstream, there is at least one important difference. Starches take longer to digest than sugars, and therefore the digested sugars enter the bloodstream more gradually. This reduces the sudden spike in blood sugar that can result from the consumption of large quantities of sugar.

Enzymes in our saliva and in our stomachs break down the long chains of starch molecules into the Lego blocks of simple sugars. They do this by snipping the links that connect the rings. However, there are actually two different ways to link together two saccharide rings. One type of link is called an alpha linkage, and other is called a beta linkage. All of the links in digestible carbohydrates — starches and 2-ring sugars — are of the alpha type. Therefore our digestive enzymes specialize in snipping alpha links, and lack the ability to snip beta links.

Cellulose, an important component of plant cells, is another polysaccharide — very similar to starch — that consists of thousands of simple sugar rings linked into a long straight chain. However, all of the links in cellulose are of the beta type, and therefore we don’t know how to digest it. On the positive side, cellulose adds “bulk” to the material that passes through our digestive system, which is very helpful for maintaining healthy intestines. Although humans cannot digest cellulose, certain kinds of bacteria are able to snip the beta linkages, releasing the simple sugars. The stomach of a cow includes a fermentation tank (called a rumen) that harbors such bacteria, which allows the cow to unlock the nutritional value of high-cellulose foods such as grass.

In summary, the Lego blocks for carbohydrates are simple 1-ring sugars such as glucose and fructose. Certain other sugars, such as table sugar, consist of two Lego blocks. Starches consist of hundreds or thousands of Lego blocks. Sugars and starches that are completely digested into individual Lego blocks are allowed to sneak through the walls of the small intestine and enter the bloodstream. But where do these simple sugars — the carbohydrate building blocks — come from in the first place? The answer is that these molecules are manufactured by plants in a process called photosynthesis — capturing and storing the energy of sunlight in the form of sugar.

Fats

Fats and oils, also called triglycerides, are the second category of basic food materials — and they are composed of a completely different set of building blocks. (An oil is simply any fat that happens to be liquid at room temperature.) The unusual thing about fat molecules is that each one consists of exactly four Lego blocks — no more and no less. The structure looks a bit like a really wide letter E. Three molecules of fatty acids (the three arms of the E) are attached to a short little molecule of glycerol. These fatty acids have names such as stearic acid and oleic acid. The body insists on breaking down each molecule of fat into its four basic pieces before allowing the pieces to exit the intestine and enter the bloodstream.

The tiny little glycerol molecule is of little interest, except for its ability to link up with three fatty acid molecules. Therefore it is the fatty acids that get all the attention. A fatty acid molecule is simply a chain of carbon atoms with two oxygen atoms at one end. (The remaining bonds are occupied by hydrogen atoms.) The end with the two oxygens is called a carboxyl group (with a chemical formula of –COOH), and this is what makes the molecule an “acid”. The most common fatty acids in our food each contain 16 or 18 carbon atoms — in a single, unbranched chain. However, there are dozens of kinds of fatty acids, with lengths ranging from 4 to 24 carbon atoms.

All of the common fatty acids in our food are fairly similar, with only two distinguishing factors: 1) the number of carbon atoms in the chain, and 2) the number and location of any double-bonds between adjoining carbon atoms. For example, stearic acid has a chain of 18 carbon atoms and no double bonds. Oleic acid also has 18 carbon atoms, and it has one double bond right in the middle — nine carbons from either end of the chain.

Stearic acid. Black=carbon; red=oxygen; white=hydrogen.
Oleic acid. Note the double bond in the middle of the molecule.

Because every fat molecule is assembled from three fatty acid molecules, a single molecule of fat might contain one, two, or three distinct kinds of fatty acids. This means that there are thousands of kinds of fat molecules, each with a different combination of fatty acids. Any source of fat, whether from a plant or an animal, will contain many kinds of fat molecules. However, because all of these fats get broken down into fatty acids when we digest them, we don’t bother to identify the specific fat molecules. Instead, we identify the percentage of different fatty acids in each type of fat or oil that we consume. For example, oleic acid is the dominant fatty acid in olive oil, canola oil, and almond oil. Linoleic acid is the dominant fatty acid in grapeseed oil, safflower oil, and sunflower oil. Palmitic acid and stearic acid are also quite common, especially in animal fats.

The difference between saturated fats and unsaturated fats is that saturated fats do not have any double bonds between the carbon atoms in the chain. Animal fats tend to have a much higher percentage of saturated fats than do oils from plants. When you see the term “hydrogenated vegetable oil” in a list of ingredients, it means that the naturally occurring unsaturated fats have been subjected to a chemical process to convert them into saturated fats — making them more like animal fats. Stearic acid is the most common fatty acid in hydrogenated vegetable oil.

We all know that our bodies store excess food energy as fat. This fat is stored in special fat storing cells, and a mass of such cells is called adipose tissue. But fats also have other important purposes. For example, all of the cell membranes in our bodies are constructed in part from fatty molecules called lipids. Several important regulatory molecules, such as certain kinds of hormones, are also built from lipids.

As odd as it may seem, soaps are made from fatty acids. Soap-making has a long history — you may have read stories about pioneer families making their own soap, using animal fat and wood ashes as the starting ingredients. A soap molecule is a fatty acid in which the acid end has been joined to a sodium atom or some other equivalent. An example of a typical soap molecule is sodium stearate — and the name tells you that it is derived from stearic acid.

In summary, the Lego blocks for fats are simple linear chains of carbon atoms called fatty acids. All fat molecules consist of exactly three fatty acid molecules, connected to a tiny molecule of glycerol (also called glycerin). Fatty acids, like simple sugars, consist of only three types of atoms — carbon, oxygen, and hydrogen. The most common fatty acids in our foods have 16 or 18 carbon atoms. Some of these — such as stearic acid and palmitic acid — are saturated fats. Others — such as oleic acid and linoleic acid — are unsaturated. Fats and oils that are completely digested into fatty acids are allowed to leave the intestine and enter the bloodstream.

Proteins

Our third and final category of major food nutrients is proteins. Like carbohydrates and fats, a protein is composed of simpler units that must be taken apart during digestion. However, proteins are built from amino acids — a completely different set of Lego blocks than those used in carbohydrates or fats. Depending upon how you count them, there are approximately 20 kinds of amino acids in our foods.

A protein is usually a very large, complex molecule, consisting of thousands of amino acid units. The amino acids themselves range in complexity, with some that are even simpler than simple sugars or fatty acids. Other amino acids are larger and more complex, but still small enough to pass through the intestine wall into the bloodstream. A few of these amino acids have names that you might have heard of, such as tryptophan, lysine, or glutamine.

Although amino acids vary in size and complexity, all of them share a small core in which a central carbon atom is attached to a nitrogen atom on one side, and a carboxyl group (–COOH) on the other side. The carboxyl group is what makes the molecule an “acid”, just as we saw with the fatty acids. The nitrogen atom, attached to two hydrogen atoms, is what makes the molecule an “amine”. Therefore the molecule is an “amino acid”. The central carbon atom has two additional bonds, one of which is attached to a hydrogen atom. Therefore the only difference between various amino acids is the “side group” that connects to the fourth bond of the central carbon atom. However, this side group can range from a single hydrogen atom (in glycine) to a complicated structure consisting of up to nine carbon atoms. The side group can be straight, branched, or even include rings. It might or might not include atoms of nitrogen, oxygen, or even sulfur. The upshot is that amino acids are quite diverse.

The pattern for all amino acids. The one part that differs is the “side group”, shown here as “R”.

One truly special detail that sets proteins apart from fats and carbohydrates is the association with DNA. Our DNA specifies the recipes for building thousands of kinds of proteins. Each group of three sequential “base pairs” in our DNA corresponds to one of 20 distinct amino acids. Therefore a strand of DNA is a coded message that reveals the correct sequence of amino acids to build a specific kind of protein. Our bodies rely on proteins for all kinds of essential purposes. When most of us think of proteins, we recall their essential role in building muscles (because muscle tissue is composed mostly of protein). However, of the many thousands of kinds of proteins that our DNA contains recipes for, only a tiny fraction are used as construction materials for muscles and other tissue.

Instead, the vast majority of these proteins serve other roles. A particularly important class of proteins consists of enzymes, which act like little engineers that help to build other molecules or to take them apart. Therefore the enzymes that disassemble the food in our stomach and intestine are all proteins. Other enzymes in our blood and tissues use the resulting Lego blocks to build new things. Still other enzymes release and utilize the energy in our stored sugars and fats. In effect, our bodies are run almost entirely by proteins and by other molecules that are assembled by proteins. Therefore, our DNA only needs to contain the recipes for building proteins. Our DNA does not contain recipes for building fats, carbohydrates, or most other categories of complex molecules.

Because the recipes in our DNA require 20 specific ingredients — that is, 20 distinct amino acids — all of these amino acids need to be available wherever protein building is going on. However, we don’t necessarily need to have all 20 amino acids in our foods, because our bodies know how to turn certain amino acids into certain other amino acids. There are nine “essential” amino acids that our bodies don’t know how to make, and therefore we must get all nine of these from our foods. On the other hand, it certainly helps if we can get most of the other 11 from our foods as well.

In summary, the Lego blocks for proteins are amino acids. Unlike simple sugars and fatty acids, amino acids contain nitrogen in addition to carbon, oxygen, and hydrogen — and some amino acids also contain sulfur. Proteins that are completely digested into amino acids are allowed to leave the intestine and enter the bloodstream. Our bodies need 20 different types of amino acids to build proteins, following the recipes encoded in our DNA. Our bodies need proteins not only as construction materials for muscles, but also to create the enzymes that directly or indirectly control nearly everything else that goes on in the body. Other important molecules, such as antibodies, are also types of proteins.

Other Things in Our Food

Other than carbohydrates, fats, proteins, and water — the four largest constituents in our food — we also need certain other nutrients in much smaller quantities. We can classify most of these as vitamins and minerals. Minerals are simple inorganic materials that provide essential atoms such as iron and magnesium. Vitamins are essential organic molecules that are needed in small quantities, and that our bodies don’t know how to make, or don’t make enough of. Vitamins are small molecules that — like simple sugars, fatty acids, and amino acids — can pass through the intestine wall into the bloodstream.

In a few cases, our vitamins might come from slightly more complex molecules, requiring a small amount of disassembly. A good example is Vitamin A, which we usually ingest as beta carotene — the pigment that makes carrots and sweet potatoes orange. Our digestive system cuts each molecule of carotene in half, resulting in two molecules of Vitamin A.

Any drug that can be taken orally must also consist of molecules that are small enough to qualify for the secret exit from the intestine. Drugs that consist of large molecules must be injected instead. Other small molecules in our food provide many of the flavors and odors that we associate with our foods. Some of these, after entering the bloodstream, can provide a distinctive odor to our urine, or even to our breath. (Consider “garlic breath”, for example.)

However, none of these other examples — vitamins, minerals, orally administered drugs, or natural flavor compounds — require disassembly in the same manner as carbohydrates, fats, and proteins. None of them provide essential Lego blocks, comparable to simple sugars, fatty acids, and amino acids. Therefore, if you know about these three categories of Lego molecules, then you know the key building blocks from which most things in our bodies are ultimately built.

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